1. Field of the Invention
This invention concerns illuminating a transparent substance, such as existing storefront window pane and particularly storefront panes. It is desirable to illuminate in a cost-effective manner so as to attract attention and try to promote incremental sales.
Light (UV, visible and/or IR) from a source (e.g., LED, laser diode, optical fiber, fluorescent or photoluminescent materials) is optically coupled into a transparent substrate (e.g., storefront window) via toroidal or circular (FIGS. 1, 1A, 1B, 2, 4, 12) prism couplers. These couplers can be refractive (FIGS. 1, 1A, 1B), reflective (FIG. 4), or some combination thereof, and comprise a refractive medium (e.g. oil, gel, water, adhesive, etc) between the optic (e.g. circular prism) and the substrate (e.g. window) in order to non-invasively introduce light into the substrate at angles that cannot be introduced via air coupling. Use of such prism couplers to the face of a transparent substrate is one element of the invention disclosed herein. The light is then trapped within the substrate or window, which is akin to light trapped within an optical fiber i.e., via total internal reflection, TIR. Definitions of TIR, the ‘critical angle’ and the ‘evanescent wave’ appear in U.S. Pat. No. 5,959,777, column 2, line 56-Col. 3, line 8, and ‘optical contact’ in Col. 1, lines 55-59 in the same patent, all incorporated by reference.
Light can then be extracted from the surface of the substrate or window by defeating TIR via techniques known in the art. Examples of such techniques would be the application of scattering/fluorescing inks or the lamination of surface or bulk diffusing films to portions of the window surface
2. Prior Art
Traditional edge lighting of a transparent substrate introduces light into the substrate along at least one edge thereof, and light is trapped in the substrate and propagates along the substrate as the light is reflected off the surfaces on the inside of the substrate (see, e.g., U.S. Pat. No. 6,036,328).
FIG. 5 hereof illustrates a prior art transparent substrate or light guide which is illuminated by edge lighting, particularly an uncollimated source (LED in this example) is coupled into the edge face of a light guide, S1, separated by an air gap (n1=1.0).
FIG. 5 shows equations used in analysis of a traditional edge-lighting approach for a light guide surrounded by air (n3˜1), receiving light into its entrance surface S1 from an air-coupled (n1˜1) LED having a lambertian angular distribution. Note that FIG. 5 shows one plane through the window. The light from the LED actually is emitted in a hemisphere comprising rays R1 that would be in and out of the plane of the illustrated flat panel light guide or pane, i.e., out of the page hereof. In the analyses described herein, all calculations are for rays within the plane of the paper. One of skill in the art can determine the effects at other angles out of the plane of the paper.
FIG. 6 illustrates analysis results for a light guide of FIG. 5 having a refractive index, n2, of 1.51. Note that the light rays (for all angles) reflect off the sidewall surfaces S2 and S4 via total internal reflection (TIR). However, all angles also exit surface S3, opposite entrance surface S1. Note also that the incident angles (relative to axis AX2) at surface S2, identified as (90−θ2) are restricted to angles between 48.53 and 90 degrees. Further, the critical angle at surface S2, (90θ−2)=asin(n3/n2)=41.47 degrees, and its complement is (90-41.47)=48.53 degrees. Therefore, for the geometry shown in FIG. 5, it is physically impossible for rays to strike surface S2, relative to axis AX2, between the critical angle and its complement.
Since the rays from the LED, R1, span the entire gamut of ray angles, −90°≦90° into S1 (see column 2 in FIG. 6; only positive angles are shown), one can see that for a rectangular slab light guide (i.e., a mall window), light introduced at one end face, S1, will leak out the other end face, S3, unless a reflective material is affixed on or adjacent to S3.
An alternate edge lighting approach would be to optically couple the LED to the edge face, S1, instead of using an air gap. That would introduce all angles within the light guide, and so rays R2 would extend −90°≦θ2≦90°, and so light within a range of angles would leak out of edge faces S2 and S4. For example, for n2=1.51, rays R2 would leak out of face S2 for θ2>48.53°; i.e. the incident angle at S2 relative to axis AX2, (90−θ2) the critical angle, or 41.47°). So, to prevent leakage, reflective tape would then be placed on portions of S2 and S4 starting at point intersecting with face S1, wherein the length of the tape progressively gets longer as the LED moves along edge S1 further away from the intersection with S2 and S4, respectively.
One then might consider partially collimating the LED before optically coupling to the edge face, S1. First, a typical lens would lose its ability to collimate if its curved output face were optically coupled to the edge face S1 (unless the refractive index of the lens was very high in order to maintain the appropriate Δn). A non-imaging approach might work, with the exit face optically coupled to the edge face, S1, and the collimation set θ2<48.53°, which would preclude leakage out of face S2, although not out of S3 unless the incident angle at S3 relative to axis AX7, θ2, is greater than the critical angle or 41.47°. Note that since S2 and S3 are planar surfaces at the angle of incidence, the angles discussed above work the same when reflected about the axis being considered (these can be considered as negative angles). Further, for the ordinary case of a rectangular slab (window), there is a relationship between the critical angle at S2 and that at S3. For example, at S2, there will be TIR for angles (90−θ2)>41.47°, which can be rewritten)(90−41.47°)>θ2, and thus θ2<48.53°. At S3, TIR will occur if θ2>41.47°. Therefore, combining both constraints, TIR can occur at both S2 and S3 if 41.47°<θ2<48.53°; i.e. for θ2 between the critical angle at S2 and its complement. However, as stated previously, this range of angles is not possible for that shown in FIG. 5. Finally, note that non-imaging optics (when used with a multi-lumen LED) would have a length that may interfere with existing structures in retrofit applications, and might require additional spacing between adjacent mall glass windows in new installations (or require wider window frames for a given size exterior window). The length of the optic would scale with smaller LEDs (see general discussion of non-imaging optics in U.S. Pat. No. 5,971,551 noting that a collimator is but a concentrator in reverse), but then many more LEDs would be required to achieve the same number of lumens, all else being equal.
Second, the natural compression of angles once inside the light guide, in the edge lighted air-gap approach, −41.47°≦θ2≦41.47° (for n2=1.51, typical for glass) limits the spread of light through from the point of entry at face 51 (see e.g., FIG. 17F). Compare this with the omnidirectional spread of the instant invention shown in FIG. 17B. Therefore, for the edge lighted air gap approach, in order to fill the window with light, one would need a substantial number of LEDs along edge face 51. Further, since the span across the window can be rather large (say 4-6 feet for mall glass), then due to losses (absorption, scatter, etc) over that span, one would need to fill the opposing edge face, S3 with LEDs as well in order to provided reasonably uniform light across the window (see e.g., FIG. 17I).
By use of the prism coupling approach hereof, light can be injected at precisely the location(s) on the window where it is needed since it is not restricted to a location at the edges of the window.
Light in the window or light guide may be caused to escape the light guide, rather than being internally reflected. The escape may be caused by some treatment at selected locations on a surface of the light guide. Typical ways used to cause escape may include a film applied to a surface at selected locations, e.g., described in U.S. 2007/0279554 and U.S. Pat. No. 6,171,681, e.g., cling film and electret film, described in U.S. 2004/0043221 (the '221, referenced below). See U.S. Pat. No. 5,319,491, col. 2, line 40-col. 3, line 26, incorporated by reference, which describes additional approaches for coupling light out of a substrate.
For example, indicia on a cling film may be caused to glow, e.g., by a fluorescent substance applied to them when the film is directly mounted to the light guide and the indicia are illuminated by a light source optically coupled to an appropriate optic. Optical coupling occurs between the light guide and the cling film, even without need for an interposed coupling medium between them.